U.S. Pat. Nos. 3,122,629 and 3,163,743 disclose non-consumable electrode electric arc welding processes which utilize a so-called hot wire system. According to these processes, an electric current is passed through a wire which produces Joule heat therein, the amount being I.sup.2 R where I is the value of the current and R is the wire resistance. While the wire is thus heated, it is fed to an electric arc zone, and welding is carried out. In accordance with this process, is is possible to greatly increase the amount of welding as compared with the process where the filler wire is fed without being heated as described above. However, the process where the wire is separately heated requires two relatively large torches--i.e., a torch for the generation of an electric arc and a torch for heating the filler wire; and therefore, this process is not suitable for hand welding.
In order to make possible hand welding according to the above-described wire heating process, it is necessary to combine the two torches together in one unit and to miniaturize the torches as much as is possible for practical use. To achieve maximum miniaturization, it is desirable to employ a structure where a wire-heating torch is fixed at an attitude which is parallel to a TIG (tungsten inert gas) torch, so that only a top portion of the wire is inclined toward the TIG torch. Thus, there have been proposed apparatuses as shown in FIGS. 1 and 2.
In the conventional apparatuses of these figures, an electrode 2, made of tungsten, is held in a TIG torch 1 coaxially. An arc 6 and a weld puddle 28 are shielded by a shield-gas 7 from a shielding nozzle 3. A wire-heating torch portion is secured to the TIG torch 1 by means of a holder 4. A sliding piece 8 is designed so that it moves up and down in a cylindrical slide base 9 which is fitted firmly to the holder 4. An adjusting nut 12 is positioned in a notched portion of the slide base 9 and is in engagement with a big screw formed in an upper portion of the sliding piece 8. Therefore, when the adjusting nut 12 is turned, it does not move up and down, and accordingly the sliding piece 8 travels up and down. A guide groove is provided in the sliding piece 8 for preventing rotation, and a knob 11 fitted to the slide base 9 is engaged with the guide groove. A guide 27 is screwed to a lower portion of the sliding piece 8 in such a manner that the guide's extended line is directed toward the electrode 2. An insulative guide 26 is fitted into the interior of the guide 27, and similarly, an insulative tube 10 is fitted into the interior of the sliding piece 8. A small diameter hole is bored in the insulative guide 26 and the insulative tube 10 which is sufficiently large so that a wire 5 can pass through the hole. The wire is fed to the arc zone 6. A conduit cable 20 is fitted in the center of a power supply device 18, and the other end is connected to a wire-heating power source 21. A power supply tip 13 is screwed to the power supply device 18 through an insulated cap nut 14. By screwing the cap nut 14 to the sliding piece 8, the power supply tip 13 and the power supply device 18 are attached electrically while being insulated from the environment. The power supply device 18 is covered with an insulative tube 19.
The electric arc 6 is generated between a workpiece 22 to be welded and the electrode 2, and the weld puddle 28 is formed by the arc heat.
Next, the operating procedure of the abovedescribed conventional apparatus is described.
First, the adjusting nut 12, screwed to the sliding piece 8, is turned to move the sliding piece 8 up and down until the location of the end of the wire 5 is positioned properly with respect to the electrode 2. The wire 5, fed into the conduit cable 20 from a wire-feeding apparatus (not shown), passes through the power supply tip 13, and is sent to the weld puddle 28 by means of the insulative tube 10 and the insulative guide 26. A wire-heating current is supplied from the wire-heating power source 21 through the copper wire 17 and the power supply device 18 to the power supply tip 13. A lower end of the power supply tip 13 supplies the current to the wire 5. Since the wire 5 is insulated along its length by the insulative tube 10 and the insulative guide 26, the wire-heating current flows through the wire 5, the weld puddle 28, and the workpiece to the wire-heating power source 21. At this time, the wire 5 is heated to near its melting point by the I.sup.2 R heating caused by the resistance (R) between the lower end of the power supply tip 13 and the weld puddle 28 and the current (I) flowing therethrough. Thus, the wire 5 is easily melted by the I.sup.2 R heating in combination with the heat from the arc 6 and the heat conserved by the weld puddle 28, and the melted wire is deposited on the workpiece 22. Since, as described above, the top end of the wire 5 is heated to near its melting point, it is required for the insulative tube 10 and the insulative guide 26 to be made of material having heat resistance and insulative properties (e.g., ceramics).
In passing through the wire-guide hole of the insulative tube 10, the heated wire 5 comes into contact with a portion A of the insulative tube 10 (FIG. 2), and thus, the heated wire 5 enters the wire-guide hole of the insulative guide 26 while contacting the portion A. When the wire 5 contacts the portion A, a contact reaction results, which prevents the wire 5 from smoothly entering the inclined hole provided at a wire insert angle of .theta..degree. in the insulative tube 10. Therefore the wire again contacts the insulative tube 10 at a second portion B as shown in FIG. 2. The coefficient of friction between a wire in a heated condition, i.e., a "hot wire", and the insulative tube 10 made of an insulative material, such as a ceramic, is several times that between a wire which is not heated, i.e., a "cold wire", and the ceramic insulative tube 10.
Since the conventional hot wire type electric art weld-torch is designed as described above, the heated wire is abruptly bent at a bent portion of the wire-guide path. This increases the resistance encountered in feeding the wire and makes it difficult to feed the wire continuously and stably. When the diameter "d" of the wire-guide path is increased, although the resistance encountered in feeding the wire drops, the wire always comes into contact with the tube 10 at at least two points located at the portions A and B. Therefore, the greater the diameter "d", the more readily the wire moves in a zigzag direction through the path. Thus, many difficulties are encountered in feeding the wire continuously and stably.